Organic photo-material of increased contrast

ABSTRACT

A PHOTOSENSITIVE COMPOSITION IS DISCLOSED YIELDING A COLOR IN LIGHT-STRUCK AREAS AND COMPRISING AN N-VINYL COMPOUND AND A POLYMERIZATION INHIBITOR. THE INHIBITOR INCREASES GAMMA, OR CONTRAST, BY REDUCING PHOTOSENSITIVITY TO LOW LEVELS OF LIGHT WITHOUT SIGNIFICANTLY DECREASING PHOTOSENSITIVITY TO LEVELS OF LIGHT EXPOSURE USED TO MAKE THE IMAGE EXPOSURE.

Allg. 17, YO5|||KA2U YAMADA E TAL ORGANIC PHOTO-MATRIAL OF INCREASEDCON'IRAST Filed March 14, 1968 INVENTORS.' yam/KA zu )fx/1404, BY fsrfeEM. Sme/4 nted States Patent office 3,600,179 Patented Aug. 17, 1971ABSTRACT F THE DISCLOSURE A photosensitive composition is disclosedyielding a color in light-struck areas and comprising an N-vinylcompound and a polymerization inhibitor. The inhibitor increases gamma,or contrast, by reducing photosensitivity to low levels of light withoutsignificantly decreasing photosensitivity to levels of light exposureused to make the image exposure.

BACKGROUND OF THE INVENTION (1) Field of the invention The field of artto which the invention pertains includes the field of photographicchemistry, particularly lightsensitive compositions.

(2) Description of the prior art A variety of organic photosensitivesystems are known in the art and generally involve two or more organicmaterials which react under the influence of actinic light to produce acolor. As early as 1921, Murray C. Beebe and his co-workers describednumerous organic photographic systems utilizing halogen compounds incombination with a second ingredient (eg. U.S. Pat. Nos. 1,574,357;1,574,- 358; 1,574,359; 1,575,143; 1,583,519; 1,587,269; 1,587,- 270;1,587,271; 1,587,272; 1,587,273; 1,587,274; 1,587,- 274; 1.604,674;1,618,505; 1,655,127; 1,658,510; and 1,820,593). It has been theorizedthat light effects the release of a radical from the halogen compoundwhich carries out a color-forming reaction with the second compound.Subsequent workers such as Eugene Wainer (e.g., U.S. Pat. Nos.3,042,515; 3,042,516; 3,042,517; 3,042,518; 3,046,125; and 3,056,673) asWell as a number of other workers, have continued the development ofvarious photographic systems involving a photo-energized reaction of acombination of a halogen containing compound and one or more othercompounds. Other recent disclosures include British Pat. No. 917,919 andBelgian Pat. No. 516,094.

More recently, it has been discovered that some of the abovecombinations can be advantageously dispersed in a continuous phase inwhich the combinations are substantially insoluble (e.g. gelatin orother, similar aqueous support). When an N-vinyl compound is used as thecolor former, particularly in combination with a photo-initiator,markedly superior speeds result. This discovery has been described inBritish Pat. No. 1,065,548.

With the foregoing N-vinyl compounds and photoinitiator, particularly inthe form of a dispersion, an image-wise exposure can cause bothcolorless polymerization and color formation. The two reactionsapparently compete so that substantial gray scale is obtained undernegative-mode conditions, that is, under conditions yielding a color inlight-struck areas. It is desirable to reduce such gray scale when usingthe photosensitive material to reproduce line copies. In this utility itis preferred to use a material having a high gamma characteristic curve,i.e., high contrast, so that the background areas are less sensitive tolight and therefore remain cleaner.

SUMMARY OF THE INVENTION The present invention provides photosensitivematerial which utilizes a color forming N-vinyl compound but which hasincreased gamma or contrast. Gamma is increased by incorporating apolymerization inhibitor, particularly in a concentration sufficient todecrease the photosensitivity of the material but insufficient tosignificantly decrease image color density. In a particular embodiment,the polymerization inhibitor is an aromatic amine.

DESCRIPTION OF THE PREFERRED' EMBODIMENT Referring to the drawing,illustrative characteristic curves 10 and 12 are shown which are H & Dcurves, as known in the art, in which the density is plotted againstlogarithm of exposure, in this case the relative logarithm of intensityof exposure. Data points are obtained by exposing the photosensitivematerial through a step wedge having 11 steps, the first step beingnearly transparent. A densitometer is used to read the developed imageand to read the developed image and to obtain the points which areplotted in the figure.

Utilizing a photosensitive material containing an N- vinyl compound, butwithout a polymerization inhibitor, data points are obtained and plottedas small circles 14 to form the characteristic curve 10 of the material.Utilizing the same material but with a polymerization inhibitor inaccordance with this invention, data points are obtained and plotted assmall xs 16 to form the characteristic curve 12 of the improvedmaterial.

Comparing the curves 10 and 12, we see that by incorporating a,polymerization inhibitor the toe of the curve shifts significantlyrepresenting a significant decrease in photosensitivity to low levels ofexposure or light intensity. On the other hand, the maximum density doesnot decrease. The result is an increase in the slope of thecharacteristic curve.

Gamma represents the degree of contrast of a developed i photographicimage and is determined by measuring the slope of the straight-lineportions of the characteristic curve of a photographic material. In thedrawing, straight dashed lines 22 are drawn through the characteristiccurves 10 and 12 to intercept the abscissa 24. Gamma for the curves canthen be obtained by determining the tangents of the angles of suchinterception. By extending the upper portion of the curves, e.g. at 18,to intercept the ordinate 20, one can approximate the maximum densityobtainable under the conditions of exposure.

The N-vinyl compound utilized in our invention can be any of a varietyof N-vinyl compounds known to the art to yield a color when incombination with a photoinitiating compound or when otherwise formulatedto be photosensitive. Such aromatic N-vinyl compounds as are found inthe aforenoted British Pat. 1,065,548 can be utilized here, for example,N-vinyl phthalimide, N-vinylcarbazole or N-vinylindole. N-vinylcarbazoleis particularly efiicacious.

As polymerization inhibitor, one can use any of the wide variety ofdifferent types of inhibitors for vinyl polymerization known to the art.Such inhibitors include phenolic compounds, quinones, nitroaromatics,aromatic amines, and the like, and miscellaneous other materials. Forthe purpose of this specification and the appended claims, the termaromatic amine is meant to include N-heterocycles, such asphenothiazine. The aromatic amines are particularly effective.

When a photo-initiator is used, it is generally a compound whichproduces free radicals or ions upon exposure to light of a suitablewavelength, such as the haloalkanes, halomethylnaphthalenes,haloketones, halogenated acids,

sulfonyl chlorides and aroyl peroxides. Such of these materials as arediscussed by McCloskey and Bond in Industrial Engineering Chemistry,volume 47, page 2125 et seq. (1955), are of interest here, and suchdisclosure is incorporated herein by reference. Other materials includearomatic hydrocarbon photo-initiators such as found in U.S. 2,902,421;substituted benzoins described in U.S. 2,722,512; metal mercaptides asdescribed in U.S. 2,738,- 319; benzene thiols as described in U.S.2,861,934; ethylenic compounds as described in U.S. 2,716,633; andthiruam monosuldes as described in U.S. 2,861,933, the disclosures ofthe above patents being incorporated herein by reference.

In general, the organic halogen compounds are particularly effectivephoto-initiators and such compounds are described in detail in theaforementioned British Pat. 1,065,548. Of these, a particularlyeffective group of compounds are those in which there is present atleast one active halogen, selected from the group consisting ofchlorine, bromine and iodine, atached to a carbon atom having not morethan one hydrogen atom attached thereto. Examples of such compoundsinclude carbon tetrabromide, bromotrichloromethane and 1,1,1 tribromo-2-methyl-2-propanol. Other compounds can be found in the noted patentsissued to Eugene Wainer and Robert Sprague such as U.S. Pats. Nos.3,042,515; 3,042,516; 3,042,517; 3,042,518; 3,042,519; 3,046,125;3,056,673; and 3,082,086, the disclosures of such patents beingincorporated herein by reference.

In a further embodiment of this invention, the photosensitive materialis in particulate form. Preferably, the material is dispersed in asubstantially non-solubilizing continuous phase which is generallywater-penetrable. Such dispersions are particularly well-suited forcontrast control in accordance with this invention. The solidfilmforming component used to achieve the continuous phase may be any ofa number of generally photographically inert materials, which are, inmost cases, soluble in Water or so nely dispersible therein in theconcentrations of use that for practical purposes there is nodistinction between solution and dispersion for these materials in thecontinuous phase. Such materials include the starch and starchderivatives, proteins, (i.e. casein, zein, gelatin, thiolated gelatin,etc.) alginates, gums, and the like materials which are generallyconsidered to be natural derivatives of natural film-forming materials,any one of which in its conventional water soluble form is usable infilms of the instant invention. In addition synthetic watersolublelm-formers are particularly suitable binders for photosystems of thisinvention and such materials include polyvinyl alcohol, commerciallyavailable water-soluble polyacrylics or acrylates (i.e. water-solublepolyacrylic acid salts having substantially the molecular weight andwater compatibility of the polyvinyl alcohol), various commerciallyavailable amine or amine-aldehyde resins, etc. Also, a number ofcellulose derivative lm-formers may be used, and these include thevarious water-soluble cellulose ethers, carboxymethyl-cellulose,hydroxypropylmethylcellulose, etc. Essentially these materials arephotoinsensitive and their principal function is that of forming thedesired lm to retain the dispersed phase in discreate particle form. Ofthe above materials, gelatin, casein, polyvinyl alcohol, gum arabic,starch, alkali metal carboxymethylcellulose (eg. sodiumcarboxymethylcellulose) and hydroxyethylcellulose are particularlyuseful in this invention.

In addition to the foregoing components, a sensitizer can also be addedin small sensitizing amounts either to create a variety of colorresponses or to impart panchromatic response to the film. For a suitablesensitizer reference can be made to a 1957 publication of Brooker andVittun entitled, A Century of Progress in the Synthesis of Dyes forPhotography in the Journal of Photographic Science, volume 5, 1957,pages 71-78, the disclosure of such publication being incorporatedherein by reference.

4 All of these materials tend to absorb actinic energy and all haveexhibited their ability to transmit the absorbed energy by a physicalmolecular contact or by engagement to other molecules receptive ofactinic enery for purposes 5 of photo-response. Illustrative materialsare: compounds having a characteristic rhodamine ring structure andnominal substitution as an amino group, such as the rhodamines (i.e.aminophthaleins); materials having simple (unsubstituted) carbon chainsinterconnecting fused heterocyclic and benzenoid ring structures, suchas pinacyanole and related carbocyanine or cyanine type dye bases; lightabsorbers containing heterocyclic rings fused with benzenoid rings, suchas eosin dye bases; and light absorbers having a triphenylmethanestructure particularly leuco forms thereof and compounds havingthia'zne, alizarine, acridine and anthraquinoid groups of structures.

In general, light absorbers having a styryl substituent such as thestyryl azastyryl dye bases are particularly useful in our invention.

The concentrations of the foregoing components depends on the particularphotosensitive composition utilized. Generally, a weight ratio ofN-vinyl compound to photo-initiator of from about 1:5 to about 50:1 issatisfactory. A concentration of N-vinyl compound in the binder can varyfrom as low as 5% of the binder, or lower, to where no binder is used insome embodiments. When a sensitizer is used, generally from about 0.001to about 100 mg. of sensitizer per gram of N-vinyl compounds issatisfactory.

The polymerization inhibitor is present in a concentration suicient todecrease photosensitivity of the material but insufficient tosignificantly decrease the density of developed color. The amount variesin accordance with the nature of the photosensitive material as well asthe nature of the polymerization inhibitor. Generally, for moreeffective inhibitors such as the aromatic amines, as little as 0.001 mg.and as high as 100 mg. can be used. Other, less effective, inhibitorsmay require as high as 500 mg. or even higher to be effective.

The following examples illustrate the variety of polymerizationinhibitors effective utilized in this invention.

EXAMPLES 1-4 Baryta paper was coated to about 1.5 mil wet thickness withemulsions of the following formulations:

TABLE I Examples Component Amount Gelatin. 10 grams. All i Water 36 ml.

N-vmylcarbazole 4grams All Carbon tetrabromide 3.2 grams All4-(p-dimethylaminostyryl)-quinoline 4mg. All. 1.5% aqueous Tergitol 4(Union Carbide). 10 drops. 1- Phenothiazine .10

lA-benzoquinoue In preparation, the N-vinylcarbazole was dissolved in 2ml. of acetone and the 4-(p-dimethylaminostyryl) quinoline was addedthereto, the solution being maintained at about 43-45 C. The carbontetrabromide was added to the acetone solution under a red safe-lightand stirred to dissolve. This solution was then added to a solution madeup of the water, gelatin and Tergitol 4, the combination beingmechanically stirred with a high-shear mixer for about 5 minutes toyield an emulsion with an average particle size of about 8 microns.After coating with a standard vacuum plate and coating bar, the sheetswere dried for about 10 minutes in an oven with forced air circulationat about 30 C. The dry coating weight was about 1.8 grams per squarefoot.

Each sheet was exposed behind a step wedge, as in the previouslydescribed procedure, to a 650-watt Sylvania Sun Gun at 14 inches for 5seconds. The step wedge has 11 steps ranging in density from near zeroto just over 3, in steps of approximately 0.3. The sheets were thenwarmed for about 15 seconds on a rotating drum heater at about 70 C. toform an image varying in color intensity in correspondence to thevariation of density of the step wedge spaces.

The sheets were stored in the dark and denstometry readings wereobtained soon thereafter. The images can be stabilized against lightdeterioration, e.g. by bathing with an aqueous solution of sodiumsulfte, but such procedure somewhat affects the image color.Accordingly, densitometry readings were obtained without stabilizing theimage and because the image was lacking in stability to light, a redCorning 2-59 filter was placed over the reading light in the reflectanceunit of the densitometer. This reduced the sensitivity of theinstrument, but there was still sufficient range available to readreflection densities to 1.5. The densitometer was a standard WelchDensichron l, with reflectance unit Catalog No. 3832A.

By placing a negative sign in front of the step wedge densities, arelative logarithm of intensity number is obtained that is analogous tovariations in exposing light and Iwhich is useful in plotting andevaluating data. The data were plotted under standard H & D proceduresin which the density of the example is plotted against the logarithm ofexposure; in the present case intensity is used rather than exposurebecause the figures are available from the densities of the step wedge.

Curves were obtained which are generally similar to the curves describedwith reference to the drawing and having generally the same maximumdensity for the control sample as for each of the examples. The slope ofeach curve was measured to determine gamma. The following Table II liststhe results of such data analyses.

Referring to Table II, We see that each of the polymerization inhibitorscaused an increase in gamma or contrast. One may also note that a widerange of concentrations are utilized, depending on the material. In eachcase, the concentration represents :a range of concentration that hasbeen found to be effective for that material.

6 In general, the aromatic amines, as exemplified herein byphenothiazine and tetramethylbenzidine, are generally more effectivethan other types of inhibitors.

It will be understood that modifications and variations may be effectedwithout departing from the scope of the novel concepts of thisinvention.

What is claimed is:

1. A photosensitive formulation, comprising:

a composition responsive to light yield a color in lightstruck areas andcomprising a polymerizable aromatic N-vinyl compound, an organic halogencompound, 'which produces free radicals or ions upon exposure to lightof a suitable wavelength, as a photo-initiator for said N-vinylcompound, and a polymerization inhibiting compound, selected from thegroup consisting of phenolic compounds, quinones, nitroaromatics andaromatic amines, in a concentration suicient to increase contrast ofsaid material but insufficient to significantly decrease the density ofsaid color, said N-vinyl compound, organic halogen compound andpolymerization inhibiting compound being dispersed in a hydrophilicbinder.

2. The material of claim 1 wherein said polymerization inhibitor is anaromatic amine.

3. The material of claim 1 wherein said N-vinyl compound isN-vinylcarbazole.

4. The material of claim 1 wherein said polymerization inhibitor isphenothiazine.

5. The material of claim 1 wherein said polymerization inhibitor is anN-heterocycle.

References Cited UNITED STATES PATENTS 2,993,789 7/1961 Crawford96-115PX 3,042,518 7/ 1962 Wainer 96-90X 3,147,117 9/1964 Wainer 96-90X3,245,796 4/1966 Burg 96-115PX 3,279,919 10/1966 Laridon 96-115PX3,445,229 5/1969 Webers 96-35.1X

NORMAN G. TORCHIN, Primary Examiner J. L. GOODROW, Assistant ExaminerU.S. Cl. X.R. 96--115P

